1. Field of the Invention
The invention relates to an electric power steering device used in a vehicle such as an automobile or the like.
2. Description of the Related Art
An electric power steering device of so-called direct drive type, with an electric motor whose output torque is directly transmitted to a steering shaft without the intervention of a reduction gear mechanism, is described in, for example, Japanese Patent Application Publication No. 2005-145436 (JP-A-2005-145436).
An electric power steering device is equipped with an electric motor to provide assistance in steering. Generally, there is limited space within the vehicle to place the electric power-steering device; it is therefore desirable to make the electric power steering device compact. However, the output torque of the electric motor in direct-drive type electric power-steering devices is not amplified by the reduction gear mechanism, which necessitates the use of a high-output electric motor in direct-drive type electric power-steering devices. Accordingly, the motor used in direct drive type electric power steering devices is often bulky. The need to use a bulky electric motor conflicts with preference for more compact electric power steering devices.
In Japanese Patent Application Publication No. 2004-1626 (JP-A-2004-1626), an electric power steering device equipped with an electric motor coaxially coupled to a steering shaft is described. The electric motor is equipped with a rotor and a stator, and the stator is held within a motor housing. The motor housing includes a stator holding portion and a bearing holding portion that are integrally formed of a single member. The motor housing supports part of the steering shaft via a bearing held by the bearing holding portion.
For example, the stator holding portion and the bearing holding portion may be integrally formed by pressing a single steel plate. Manufacturing the motor housing in this manner reduces both the weight and cost of the housing. However, by forming the motor housing through pressing, there is a maximum limit to the thickness of the plate that can be produced via pressing. Therefore, it may be impossible to ensure sufficient strength of the bearing holding portion. In this case, the motor housing may become unable to stably support the bearing and the steering shaft due to the flexure of the bearing holding portion or the like. As a result, the wobble of the steering shaft and vibrations or noise resulting from the wobble may occur.